Transverse waves with tension

A telephone cord is 4.00 m long and has a mass of 0.300 kg. A transverse wave pulse is produced by plucking one end of the taut cord. The pulse makes three trips down and back along the cord in 0.700 s. What is the tension in the cord?

ok, this problem seems very easy and straightforward but i am getting it wrong..i have absolute no idea why ..i hate physics cant wait till i finish this damn course...

A telephone cord is 4.00 m long and has a mass of 0.300 kg. A transverse wave pulse is produced by plucking one end of the taut cord. The pulse makes three trips down and back along the cord in 0.700 s. What is the tension in the cord?

ok, this problem seems very easy and straightforward but i am getting it wrong..i have absolute no idea why ..i hate physics cant wait till i finish this damn course...

anyways....

t=time
T=tension
u=mass/length
L=total length

t=L*sqrt(u/T)

simple, and just solve for T right??? But i am getting it wrong why??

t=.7 sec
u=(.300 kg/4 m)
L= 3*(4 m)

i have all the variables but T and solve for T and get 22.03 N

but the answer is wrong does any 1 know where i went wrong thx

Yes. What does 't' represent in your formula ? You said it is the time, but it's the time taken for what ? Where did you find that formula ?

I'm also doing a similar problem.
An ethernet cable is 4.10 m long and has a mass of 0.210 kg. A transverse pulse is produced by plucking one end of the taut cable. The pulse makes four trips down and back along the cable in 0.815 s. What is the tension in the cable?